package company.beike;

import java.util.Scanner;

public class BeiKe1C {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String n = sc.nextLine();
        String s = sc.nextLine();
        int i = 0, j = s.length() - 1, count = 0;
        while (i < j) {
            if (s.charAt(i) != s.charAt(j))
                count++;
            i++;
            j--;
        }
        System.out.println(count);
    }
}
